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How does a large cargo plane like the C-130 Hercules take off and land in such a short distance?
So I was at an air show and saw a C-130J Super Hercules showcasing its capabilities.
The take-off distance it covered was amazingly short (I guess equal to fighter jets), even though it uses propeller based engines. The landing was also super short.
My questions are:

How does such a massive plane manage to take off in short distances ? Is it because it has four engines?
During landing, I couldn't see any thrust reverser (as in commercial jets) in play. What tech does it use to stop so quickly?

Thrust reversal on propeller engines (no matter whether turbine or piston) is done by changing propeller blade pitch, so its use is not very obvious.

Note, that short take-off roll is easier with propellers than jets, because jets have (approx.) constant thrust, propellers have (approx.) constant power, which means they yield higher thrust at slow speed.

surprised nobody mentioned the 727's amazing short-distance landing capabilities. According to this book I have (although i cannot find a website to confirm it), it said that it could land on 1500 foot runways

@Markasoftware near empty, 3 engines at full TR, full flaps, spoilers, and brakes... http://www.boeing.com/assets/pdf/commercial/airports/acaps/727sec3.pdf lists just under 3500ft minimum however (pg.25) and that's the official Boeing documents. Would possibly be shorter given extremely low weight and high headwind conditions.

C-130 landing, taking off, and doing touch & gos on an aircraft carrier: http://www.military.com/video/military-aircraft-operations/carrier-landings/c-130-carrier-landing-without-hook/2812569251001/

When the same kinds of STOL optimized design principles are applied to smaller aircraft like the DHC-6, the results are even more unbelievable. https://www.youtube.com/watch?v=k_aKUdMsK7M (Not that the stunt in this video is a particularly good idea)

Takeoff and landing distance have a lot of variables.
One is certainly thrust. The plane is designed such that it has enough thrust to reach takeoff speed within the available takeoff distance. For landing, the propellers are able to reverse the thrust and add extra braking power to stop in a shorter distance.
Another variable is landing and takeoff speed. If you lower takeoff and landing speeds, it takes less distance to get going and to stop. Being a propeller plane with a straight (non-swept) wing, it is designed to fly slower than jet aircraft. This means the plane needs less angle of attack to fly at slower speeds. The C-130 also has very large flaps for this purpose, and they contribute to lowering takeoff and landing airspeeds in two ways. First, the flaps are very large, which adds more wing area and also deflects more air downwards, increasing lift. Second, the flaps are placed directly behind the engines, which allows the flaps to direct some of the engine thrust downwards, further increasing lift.
Weight is also important. While the C-130 is designed for good performance at 155,000 lb, and can fly at up to 175,000 lb, it will be much lighter in a demonstration. From an empty weight of 75,562 lb, fuel and crew will still leave it well below its typical operating weights. The lower weight means that takeoff performance will be increased by better acceleration and a lower liftoff speed. Landing performance will also be better with a lower landing speed and better deceleration.
The newer C-130J also performs better than the older C-130H. The newer engines on the J model produce up to 4,637 shaft horsepower (shp) each (though they are listed here at 4,705 shp), while the engines on the H model produce up to 4,590 shp each. The empty weight on the J model is 75,562 lb and on the H model is 75,800 lb. So with the empty weight going down by 238 lb and the power increasing by at least 47 shp, and improved propellers, the J model has better performance. The takeoff distance at 155,000 lb is 3,127 feet for the J model versus 3,586 feet for the H model. The H model can take off in 1,400 feet at 80,000 lb (probably closer to the weight in demonstrations), so the J model can probably do even better.

you forgot one that's very important during demonstrations: weight. The aircraft will be at near minimum takeoff weight, no cargo, just enough fuel for the demonstration and reserve to an alternate in case the runway fouls. Low weight==shorter takeoff and landing runs.

radarbob is correct, these are not "blown flaps". Blown flaps are when there is a separate duct that blows air directly into the boundary layer over the flaps. These are normal fowler flaps. The propeller slipstream does help on take-off. Not on landing though because the engines don't produce significant thrust then.

Assault Takeoff/Landing Technique
This is a significant reason for the C-130's performance that you witnessed. "Assault" takeoff/landing speeds are lower than normal speeds, and closer to stall speed. Also there are some weight limitations. Further, the crew must be specifically trained.
Stopping Quickly
Any idea of a smooth landing is thrown out the window. The intent is to "plant" the wheels at about 300 feet down the runway. That is, transition immediately from approach attitude to actually landing rather than assuming a touch-down attitude and letting the aircraft float down to the runway.
The nose gear is pushed to the ground and the instant the wheels are on the ground the props are thrown into full reverse, now pushing air forward. At the same time the pilot puts all his might into mashing the brakes. He would "stand on the brakes" as we say. The yoke is also pushed full forward which puts as much weight as possible on the landing gear. The anti-lock brake system is critical here, preventing skidding out of control and blown tires.
In Vietnam, in combat conditions, 130 pilots would put the props in reverse before touching down. If you've seen the video on YouTube or the military's own site of a C-130 landing on an aircraft carrier in 1963, they did it there too.
J's Props are a Huge factor
The performance boost of the J models thanks to the new prop and engine is amazing vis-a-vis the older models. But fundamental airframe limitations are still there. Nonetheless more blades (6 vice. 4) and the "un-ducted fan" design simply pushes more air.
Show Off Configuration
An empty airplane with a light fuel load of course maximizes the short takeoff and landing performance.
C-130 Has Fowler Flaps
Not blown flaps. Just because the props push air over the wing does not make it blown flaps. Otherwise, yes, the C-130 was given lots of flapage precisely for this purpose.

I remember a C130 ride where the pilot practiced a short field landing... it hit hard with a loud clang, sounded like a bucket of bolts being dropped, and then we were all slammed forward. A rather dramatic entrance, certainly for us in the aircraft.

the props are turbo props (more specifically the Rolls-Royce AE 2100 with 4.6k shaft horsepower) and very efficient propeller blades. This combines into a takeoff distance of 953 m at 70 ton, and at the air show they would have kept the plane nearer the empty weight (which is half that) which shortens the needed takeoff length.
To compare the Lockheed C-130 Hercules has a slightly longer takeoff distance at the same weight but only needs half that when the weight is reduced to 36 ton.
These propeller blades can change their pitch which can reverse the thrust direction, plus the wheel brakes (even large jets use mostly wheel brakes see this answer)

Propeller driven aircraft usually have a better acceleration/deceleration rate compared to similar jet driven aircraft. This, combined with a straight/tapered wing, instead of a swept wing, and large blown flaps (as stated by fooot) make an aircraft which is able to take-off and land on very short runways.


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Why does the F-111 sometimes squirt a giant fire plume behind it? (NOT afterburners!)
As seen in multiple pictures:
First, this one, it appears the aircraft is spraying copious amounts of fuel into the exhaust!
Surely this provides no thrust, not in the way an afterburner (which the F-111 has!) does.
Is it perhaps as an alternative to flares for anti IR missiles? It seems logical, but I don't think so, because I've seen it on takeoff too!

At any rate, it is QUITE badass!

EDIT: YouTube video!
EDIT: Can any other aircraft perform this?

It's not actually fuel, but US currency that it's dumping behind it.

@KeeganMcCarthy given that only the Australians do it regularly, and that only during displays, it's actually Australian currency.

The real question is why don't all planes do this?

I have been told by a former F-111 pilot that this maneuver is called "the Zippo trick."

@SkipMiller In Australia, we actually call it "The Dump and Burn"

Pilot had beans for breakfast?    parp FHOOOOOSSHSHHHHH!!!!

The F-111 has, like many aircraft, a fuel dump port so it can get rid of a lot of heavy fuel rather quickly. Most aircraft have the dump ports on the wings, the F-111 designers put it in the tail between the engines. End result is if you dump fuel and briefly light the afterburner you will ignite the liquid fuel in your wake creating a rather spectacular trail of fire.
Aerodynamically it's completely useless, financially it's very expensive, you'd only do it in combat if you were suicidal as it greatly increases your IR signature right where you don't want it, but at an airshow it does look totally badass, especially as the afterburner is adding a lot of noise at the same time.
I would expect any pilot who does a dump-and-burn outside of an airshow (or an emergency) will have his own tail set on fire by the air wing's commanding officer.
The SR-71 had a similar issue - it leaked like a sieve on the ground. Rotating for takeoff would occasionally set fire to the JP-7 on the runway leaving a burning trail behind it much like the Road Runner.
Reply to the video:
watch the takeoff section carefully. You can clearly see the afterburners come on before the fuel dump starts.
Reply to about half the comments:
Highly unlikely they designed it to do this. The F-111 was one of the earlier swing-wing planes, that meant that you can't put much on the wings. And as the bigger tank is in the fuselage anyway, someone would have looked at the space between the engines and thought it was just right for a dump pipe. The flame effect was probably discovered later.

any links to pictures or videos of the SR-71 leaving a burning trail would be awesome.

I'm confused, if it's so useless, then why is it part of the design? Surely not only for airshows?

@user50849 All aircraft have fuel dump ports. There are a variety of questions which cover the when, why, and how of fuel dumping, see http://aviation.stackexchange.com/search?q=fuel+dump

@user50849 It also has uses in an emergency, where you might find yourself above an airfield about to land, but with too much heavy and flammable fuel: you want as little fuel as possible on a crash landing, but you don't want to fly in circles to burn fuel, so you can use a fuel dump port instead.

@user50849, it's not useless to dump fuel, and it's not useless to use the afterburners, but the only reason you'd do both in conjunction is for air shows :)

Suicidal in combat?  Couldn't it be used to confuse a heat seeking missile that was already locked onto you and approaching from the front?  (Admittedly you would have to time it just right.)

@AdamDavis Many have fuel dump ports but certainly not all.

@LorenPechtel How would that help? Seems to me it would just make you a brighter target for everything, and I don't think the fire trail behind is meaningfully further behind than the plane's usual exhaust to do anything but make your exhaust even brighter and easier to track. I can only see it helping if some missile controller decided it was too bright so it must be a flare and didn't detonate, but I doubt they do that.

@Dronz I'm picturing it like a huge flare.  Just like a flare can draw in a missile it could also.  Since it's close to the engine it would only be of use if you could use it to draw the missile behind you--a missile coming from somewhere in front of you but not head on.

@LorenPechtel Hmm. Unless I'm wrong, I think heat-seekers are already designed to target a plane in front of the engine heat, and I assume many explode when they get close. Flares are different in that the flare falls away (and the pilot generally changes direction after dropping the flare, to get as far away from the flare as possible before the IR missile arrives). There is also the hope that an IR missile will fail to keep seeing the plane's heat, especially from the front, so an ignited fuel dump would void that hope.

@casey You're right, I should have said "most"

@LorenPechtel only hypothetical case I can come up with is if the missile were to end up flying through the cloud of burning jet fuel and getting damaged as a result. But how likely is that (I'd guess missiles to be more sturdy than that, plus the chances of it happening would be miniscule).

@jwenting Who cares if it's damaged?  It can't turn around if it misses.  In reality it's going to detonate once it's flown through the cloud but hopefully it's far enough away at that point that the plane isn't taken out.  (And the fact that the plane & missile are moving apart at multiples of the speed of sound will rob the shrapnel of much of it's energy also.)

"Dumping fuel" in emergencies is useful, but how is "dump-and-burn" useful in emergencies as you put it?

@LorenPechtel, heat-seeking missiles home in on the exhaust plume and then, like all anti-aircraft missiles, explode in a shaped explosion designed to take out a nearby airplane.  A dump-and-burn will just make the exhaust easier for the missile to track.

@Mark It depends on where the missile is coming from.  If the missile is coming from the front and goes for the plume that leaves it behind the plane when it realizes it's not going to hit and self destructs.

"The F-111 was one of the earlier swing-wing planes, that meant that you can't put much on the wings."  The F-111 had four pylon store mounts on the wings, each capable of holding up to 5000lb of ordnance.

There was at least one occasion when the dump-and-burn was used in combat. During the 1986 attack on Libya

the U.S. pilots, desperate for fuel, jostling with one another in the air to get access to aerial refueling tankers. Unable to break radio silence, they could not locate the giant KC-10A Extenders and KC-135R Stratotankers that would keep them from crashing into the sea. An F-111 pilot finally solved the problem by doing a "little torching," as he dumped some fuel and ignited it with his afterburner, "creating a huge explosion that both lit up the sky and pointed the direction to the tanker"

@reirab though technically it wasn't done IN combat. It was done post-combat during a combat mission. But maybe I'm splitting hairs here.

For what it's worth, this use in combat is less suicidal than breaking radio silence.

@corsiKa out of interest, why do you say that? Does breaking radio silence make it easier to get some sort of lock?

Breaking radio silence might get you more in trouble with YOUR side, not the enemy.

Sorry to ask but how dump and burn help a fighter pilot find a tanker when they cannot break radio silence?

@vasin1987: I assume this was during the night, so they couldn’t see the tanker. They briefly turned on the lights that way.

I used to be an F111 test engineer; the jet can hold about 19000 liters of fuel (without external tanks).  At 0.81kg/Liter, that's 15390Kg (33,939 Lbs) of weight, which is substantial (the empty weight of the jet is 45200 Lbs / 20,500 Kg).  Basically with a full load of fuel, the jet is 75% heavier.
Although the giant flame plume is cosmetic, the F111 can dump fuel like this in case you need to quickly reduce the weight of the aircraft; in-flight emergencies are one possible reason to reduce weight quickly. The fire is because the pilot lit the afterburners whilst dumping fuel. Assume you loose your flaps, or maybe you discover the brake lines are leaking... if you had to land on a short field (such as a small civilian airport), it makes sense to dump your fuel so you have a better chance of stopping quickly.

I am interested, how long can an aircraft sustain this? How long can he dump fuel before he doesn't have enough to land?

@HCBPshenanigans You can land an aircraft with no fuel, so I guess the answer to the question in your comment is "until the tanks are empty."

@reirab not really. There might be technical limits to the length of time the fuel dump valve can be opened. And the burning trail of fuel might possibly damage the aircraft, so there might be a time limit there as well to prevent flames from reaching back into the fuel dump system for example.

@jwenting I was referring more to the 'How long can he dump fuel before he doesn't have enough to land?' question. Unless you're outside of glide range from somewhere to land, there's not really a minimum amount of fuel needed to land, since you can land with no power. That's not to say it's a good idea, but it is possible. Since this answer was more referring to conventional fuel dumping (as opposed to setting the dumped fuel on fire with afterburners,) I was answering more with that in mind. There might be a limit for actually setting the fuel on fire.

We did have one "practical" application in "torching" as we used to call it:  It was great way to find a Lead aircraft in a night formation to complete a night rejoin.  Also we often considered it as a useful combat technique that if we got attacked by another fighter we would plan to start a dive towards the ground and we would do a short torch just before leveling off since the flash would clearly distract the attacker and make them think we hit the ground and break off the attack.

This does not attempt to answer the question (although it is cool information that would be appropriate for a comment).

Looks to me like this absolutely answers the "why" question as stated.

@RalphJ the way I see it, this answer provides lots of cool information but if every other answer was deleted, a viewer would not have their question of “what is it doing” answered.  Since this is the case, it would be more appropriate as a comment because it helps improve the question.

The "Dump and Burn" as it was called here in Australia was used for 'effect' at air shows and for 'promoting' the Air Force as a great place to be.
The 'effect' was really quite spectacular. And frightening to those who saw it for the first time, like here in Brisbane, resulting in many phone calls to the police of a OFO sighting.
As used by some American pilots in SVN on missions to light up the area they were flying through too, for a better idea of just how close to the ground they were.

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Why does the B-2 Spirit have a pattern of thin white lines?
Northrop Grumman's B-2 Spirit strategic bomber is mostly black or dark grey, but has a symmetric pattern of distinctive, thin white lines painted on its wings and fuselage. I have not been able to find anything similar on other airplanes. Why does the B-2 have these lines and what is their purpose?

At a guess, they're 'walk here' lines, to show where you can walk, with the rest of the airframe being 'don't walk'.

Correct. Like the other decals (serial numbers, Star and bars, panel outlines), they're light gray to reduce visibility at a distance compared to a bright color like white.

they're light gray to reduce visibility at a distance good thing too. We wouldn't want those pesky enemies knowing which parts of our wings are safe to walk on!

@dwizum Probably more relevant for the CUT HERE instructions on the cockpit

I guess to merely climb in to the aircraft as a pilot, you have to use those ??

@Fattie, crew access to the B-2 is via a ladder underneath. See (e.g.) https://twitter.com/Whiteman_AFB/status/1115686738582745088

@dwizum: I realize you're joking, but to put things in perspective: back in the '80s (when I was in the Air Force) they were working on improving camo on F/B-111's, but the F-15 pilots were seeing them when they were sure they shouldn't. Eventually, an F-15 pilot admitted that they couldn't see the plane--what they could see was the pilot's shiny white helmet. Shortly after that, new helmets were also flat grey, and they issued camouflage tape to cover existing helmets.

@JerryCoffin that sort of thing is also why some air forces paint "fake" cockpits on the underneath of their aircraft, literally just to confuse an adversary for a split moment (and such moments can make or break you in a dog fight)...

@JerryCoffin yes it was a joke - great perspective in your story. The whole topic is pretty fascinating, and it's interesting to see camo and marking practices evolve as the nature of warfare changes over time.

Military aircraft, believe it or not, have relatively thin sheets of aluminum, titanium, composite, or honeycomb panels for much of the airframe. To prevent these more fragile areas from being damaged, the wing is marked with where it is and isn't safe to walk. As for the white footwear, these are typically just the equivalent of the disposable headgear you see doctors wear when they go into surgery on any doctor show on TV. They're to protect the paint, to prevent scuffs, and to prevent damaging any special paint coatings such as radar absorption (stealth).

It's unclear from the photos – is the wing generally unsafe except for the marked safe areas, or is it the opposite?

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Is it okay to take off with weight more than the POH max gross weight?
I wonder if I could take off with weight more than the gross weight stated in the POH of my aircraft as long as I take off with a higher speed?
I think I have done it once by mistake.

I think you will find your takeoff occurs at a similar speed, but it takes longer to accelerate to that speed, and your rate of climb will be much lower.  Hence you run the risk of running out of runway, or not being able to clear obstacles once airborne.  Or not being able to stop on the remaining runway if the takeoff had to be aborted.  All made worse by hot weather when engine performance is reduced.  For long overwater ferry flights, the planes are often equipped with ferry tanks to provide extra fuel, and are allowed to takeoff at over gross weight with some approval process needed.

I disagree with this a bit @CrossRoads, stall speed and climb speeds will increase with weight.

My rate of climb always seems to be lower when I am heavier, once I get to the same airspeed.  What are you calling climb speed?  I didn't mention stall speed.

@CrossRoads ...and your rate of climb will be much lower  It won't necessarily be much lower, but it will be proportionally lower.  Taking off at max gross plus one kilo won't be materially different from taking off at max gross, for example.

Vx and Vy are calculated using weight @skylover. They vary with weight, not by that much in a Cessna 152 but they do make a difference. Stall speed as well.

Another big issue is ground-effect, you'll think you can fly and you'll get the wheels off the ground, but once you try to climb out of ground effect you'll notice very quickly that the aircraft doesn't climb.

Can you take off over gross weight? Yes, it's possible on any airplane, dependent on how far over the weight limit you are, the density altitude, how much runway you have, what obstacles are in the area and other factors. It's very easy to go above gross in a Cessna 152 with full fuel and 2 passengers, and probably happens more often than people realize or own up to. That doesn't make it a good idea, which is why pilots are trained to do weight and balance plus performance calculations before every flight.
Besides the safety angle, taking off over gross weight could land you in trouble with the FAA even if nothing goes wrong, you can lose your ticket for that. If there was an incident your insurance may not cover damage or litigation costs.

This pretty much boils down to, Can you? Yes. Should you? No.

Close @FreeMan, more Can you? Maybe. Should you? No.

What is the meaning of "how far over you are" - does it mean "by how much your weight is above the maximum limit"?

Yes, that's what it means @CopperKettle.

Are you a qualified Test Pilot with insurance covering you for crashing this aircraft? The Max Gross Weight quoted in the POH tells you how far the factory Test Pilots have taken the airplane - any further and you are doing the experimenting. (An obvious concern is that with too much weight, the wings will fold in flight. But there are other concerns.)
There are margins, but you don't know what they are. So you might already have been flying within those margins, and thus got away with it. But if you don't know what the margins are then you can't guarantee to stay within them.

This is generally incorrect.  The POH is much more conservative than anything test pilots would have pushed the aircraft to and would incorporate engineering data, test flight data, and appropriate safety factors to come up with those numbers.  The POH guidance is definitely not the maximum limit a plane has ever been flown to.

Probably more accurate to say that the limit stated in the POH is the maximum that the manufacturer is willing to guarantee it'll handle...

Downvoting because of the incorrect assertion that the test pilots never took the plane beyond the POH. @anaximander suggests a very good modification to the original answer. Happy to upvote this answer instead, as it's a good one in spirit and tone, but it also has to be factually correct!

One thing that hasn't been mentioned yet is CG. Often CG is what will get you. On many aircraft the places that you are putting the weight has considerable arm from CG. This means that you are also shifting the CG. I have seen for instance Cessna 402 loaded amazingly over gross with the load as far to the front as possible. However Google Aaliya death and you will see what happens when the CG is too far aft. All in all it's a bad idea to fly over gross, and it is death to fly out of CG especially aft.

This was my thought. I would say that going out of CG is much worse than going over weight. And when you're over weight, you're more likely to significantly affect the CG.

The first sentence could just say Center of Gravity. Perhaps this is a recognized acronym, but I didn't recognize it immediately.

@FaheemMitha I think in the context of aviation everyone should probably know what CG means.

Physically, yes the aircraft can take off at or above MTOW - and depending on the aircraft and ambient atmospheric conditions, can takeoff and fly at a considerably greater weight than MTOW.  The problem becomes that the values for MTOW were decided upon during design and flight test, taking into consideration a wide range of operating conditions plus design structural static and maneuvering loads on the airframe and landing gear.  Unless you are very familiar with these numbers, you can risk structural damage to the aircraft during ground roll, stability during ground movement operations, maneuvering and landing the airplane.  In addition, you have no known values for performance data on the airplane in terms of length of ground roll, takeoff distance to clear obstacles, maneuvering load limits, rates of climb and and climbing distances, stall recovery characteristics, etc.
Therefore, in loading an airplane beyond what the manufacturer states for an acceptable weight and balance envelope, you are now swimming in unknown and potentially very dangerous, waters.  Otherwise smart and rational people die all the time doing stupid things like this in airplanes.
Additionally, intentionally operating an airplane outside of its weight and balance envelope is illegal, as it violates §§91.9(a),

No person may operate a civil aircraft without complying with the
  operating limitations specified in the approved Airplane or Rotorcraft
  flight manual, markings, and placards, or as otherwise prescribed by
  the certificating authority of the country of registry.

Overloading also could easily be cited as a violation of §§91.13 Careless and Reckless Operation and §§91.7 Civil Aircraft Airworthiness.
The bottom line:  Don't try it.  You don't want to break the law or worse, find out the hard way that your guestimates on the plane's performance were incorrect.

This is a bad idea for a host of reasons, but structural failure is one of the worst. Sensitivity to gust load is increased when overweight, reducing the designers G load margin.

source
from Mountain Flying

Consider an airplane that has a maximum allowable gross weight of
  3,000 pounds. If it encounters a +30 fps gust that results in an
  additional 2-g load factor, the airplane experiences a total of 3 Gs
  load factor. Multiply the 3-g load factor by 3,000 pounds and the
  wings are supporting 9,000 pounds.
Assume the airplane is loaded to 1,500 pounds and that it is subjected
  to the same gust. With half the inertia, the gust acceleration is
  doubled, causing the airplane to experience a 5-g load factor (4-g
  force plus 1-g level flight). Multiply 1,500 pounds by 5 gs and the
  wings are supporting 7,500 pounds.
The lightly loaded airplane is subjected to 1,500 pounds less load
  when encountering the same gust. Even though the heavy airplane
  realizes less load factor, it incurs more strain. The pilot recognizes
  load factor; the airplane recognizes load.

This is mostly incorrect. Sensitivity to gusts is generally reduced with weight: a given gust will produce a certain additional force, which will translate to lesser acceleration on a heavier aircraft, while stressing the airframe just the same. However, this somewhat depends on whether you choose to fly at a higher AoA or higher speed. Then, maneuvering speed is determined by control factors and is increased with weight; basically, heavier aircraft will sooner stall than break when pulled hard. Overall, given enough runway, performance/stall will usually be your main problem.

@zeus I did not say sensitivity to gusts. I said sensitivity to gust load, the additional load you describe. A heavy aircraft flies faster and/or at a higher AoA, and will experience a higher gust load than one that is lighter. The heavy is also closer to its structural limit, so the additional load from a gust that a properly loaded aircraft could stand might break an overloaded aircraft. You can read this directly off the chart. Read the reference on AA587, which broke apart below maneuvering speed because the pilot over controlled in wake turbulence. That aircraft wasn't even overloaded.

How do you produce a certain fixed 'gust load' to compare? It only makes sense to compare the same condition, i.e. the same gust. If you fly at a higher AoA (e.g. in cruise), but still far enough from stall, the effect of the gust will be practically the same (ignoring second-order effects). Only if you decide to fly faster will the 'gust load' from the same gust increase (but the feel/acceleration will still reduce). Yet this is hardly the most significant factor: you need a super-strong gust to exceed the design load in level flight.

...Maneuvering speed is by definition the one below which you can't cause structural failure by overcontrol, so AA587 was certainly above it. That said, it is usually specified for pitch control only, whereas technically it should be defined separately for each axis (or the slowest of them explicitly defined). Twins necessarily have a lot of rudder control, so in practice they may always be at risk. Either way, this is irrelevant; the fact is, pitch maneuvering speed increases with weight (as you can find in any POH): at a given speed, a heavier aircraft is closer to stall in level flight.

@Zeus The interesting point of the article was that training on maneuvering speed is misleading, and that you can cause structural failure below this speed. Your definition, and the one from the flight training handbook, is simplified to the point where is isn't really true. "Surprisingly, to nearly every pilot, Va is not a speed which at or below the pilot is allowed full unrestricted control surface movement without the danger of structural damage or failure." AA587 was at 2000ft and climbing when it came apart, way below Va.

You should not do that; see the example of the Colt Richter who crashed his plane after overloading it.
https://www.ktuu.com/content/news/New-information-sheds-light-on-how-pilot-died-in-Willow-Lake-crash-489634791.html
http://www.ktva.com/story/38687378/witnesses-describe-pilots-takeoff-struggle-before-fatal-crash

From the linked article, Price said Tuesday that, right now and at this point in the investigation, "I have no evidence to suggest that the plane was overloaded."

Yes, which is why I linked the other article stating the witnesses' testimony.


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Mach Factor - Factor increase of thrust due to the ram effect
In the book of Corke, Chapter 7 about engine selection, he speaks about the Mach factor which is a factor larger than 1 in supersonic to take into account the ram effect.
In his case study he takes 2,39 for a cruise speed of Mach 2.1.
He mentions that there exists algorithms and books that allow to compute this Mach factor.
I am looking for a figure showing this Mach factor in function of the Mach number. I have been looking like crazy.
If anyone has sources or this type of chart, it would be great.
Thanks

Ram pressure raises the pressure level in the engine, increasing mass flow and thrust. See this answer for more, and this one discusses the pressure increase of Concorde, which flew close to Mach 2.1. However, that 2.39 factor I cannot explain.

@PainAndGain. Hi, just curious, if you were looking like crazy for an explanation, did our answer help? Cheers.

Hi @Penguin, thank you for your answer. Actually your answer, although very complete, confused me a little. How would you define the Mach Factor mentioned in Corke ? Also, I am looking for figures of existing data or algorithms. I understand the physics behind, it's existing data that is really hard to find. I also couldn't access figure 10.35. Thank you !

@PainAndGain. Hi ... to be clear, I can't see what Corke says in his book, all I know of its contents is what you have said above. I had thought his "Mach factor" was the ram pressure rise due to forward motion of the vehicle. But now reading the statement in you question ("factor of increased thrust due to the ram effect"), I now think Corke is not talking about the ram effect, but the effect of the ram effect on thrust... So, my answer below, which is only about the existence of ram effect, not how it changes thrust, is not really hitting the relevant aspect... Cheers.

It sounds like what you are describing are the isentropic compressible flow equations. See equation 6 & 7.
At Mach 2.1, these give a stagnation pressure rise of 9.14, and a stagnation temperature rise of 1.88, since air has a value of 1.4 for gamma.
But, the air does not come to complete rest (stagnation) in the intake, hence these values are not obtained.
What you need to know, is the actual Mach number at the engine inlet, which is I assume where you are suggesting is the location of a total pressure 2.39 times higher than the freestream total pressure. Typically, this is around Mach 0.4. But, since the freestream flow is supersonic, that means it will have had to go through some shock waves, to get to a subsonic speed. This means there will be pressure losses, associated with those shock waves. Normal shock waves (a shock wave perpendicular to the flow) result in higher total pressure losses than oblique shock waves. Hence, supersonic aircraft have pointed cones in their intakes to create oblique shock waves, to minimise the pressure losses. To calculate the total pressure losses from these, you need to know the geometry of these cones to know if they cause 1, 2, or maybe even 3 oblique shock waves, as the pressure loss of each shock depends on the strength of each shock wave.
Jack Mattingly, in his text book "Aircraft Engine Design" gives a good description of how to calculate the losses associated with oblique intake shock waves. It's fairly complex. See here. The figure you want is 10.35, but that wasn't shown in the free preview when I tried it, but maybe the pages they hide are random, and you will have better luck.
So, in summary, to verify the 2.39 factor, you need to know the Mach number at the engine fan inlet, and be able to calculate the total pressure losses of the shock waves, which requires info of the intake geometry.

The answer is that the ram factor represents the increase in mass flow to the engine. The increase in mass flow due to the compressible Mach number is
m_dot ~ P/sqrt(T) where P and T are the total pressures and temperatures. These can be found from isentropic relations.
(P_t/P_s)= (1 + ((gamma-1)/2)M^2)^(gamma/(gamma-1))
and
(T_t/T_s)= (1 + ((gamma-1)/2)M^2)
Therefore m_dot ~ (1 + ((gamma-1)/2)M^2)^(gamma/(gamma-1))/(1 + ((gamma-1)/2)M^2)^0.5
At Mach 2.0, m_dot ~ 5.8 which is the ram effect multiplied by the thrust at altitude.


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Why are F-15s usually taking off with afterburners?
If there's no emergency, is it a big waste of money for military aircraft to take-off with afterburners? Why are they doing it, what are the advantages?
Like here:

It's really not that much of extra fuel since it's only a few seconds.

To be sure to not use more runway than available.
It is also good to have runway to spare in case take-off has to be aborted.
Look at this image as an example from another fighter. Notice the two X-axis. The top MIL is for without afterburner and the bottom FULL AB is with afterburner. Notice how the numbers for the MIL is almost double that of the FULL AB axis. That means much (a little less than the factor difference between the two numbers) more runway required for same conditions. And in case your wondering, that number depends on runway temperature and altitude above sea-level beside from thrust setting.


text:
A C130 lands on a carrier. How much energy must be dissipated and where does it all go?

Assumptions must be described and answers must be math justified. Extra credit for plausible obscurity.
c130 carrier landing
update:
From the video the landing roll was 270ft at 85klb and 460ft at 121klb using upgraded antilock brakes and full reverse thrust.

https://en.wikipedia.org/wiki/Arresting_gear

It's just a really, really big hydraulic shock absorber.

@Steve The C130 has no tailhook.

I didn't know that ... even after being deployed on a carrier!

Way too big a math assignment for me.

Downvotes with no comments? After all the complaints about the simplicity of the other one?

The kinetic energy to dissipate is easy to calculate:
$$ E_\text{kin} = \frac{1}{2} m \times \text{GS}^2 $$
Here, the ground speed GS must be w.r.t. to the carrier deck, not the sea.
The landing weight of the C-130 must be between the Operating Empty Weight ($\sim 75 \, 000 \, \text{lbs}$) and the Maximum Landing Weight ($\sim 130 \, 000 \, \text{lbs}$). These tests were performed at $85 \, 000 \, \text{lbs}$ and $121 \, 000 \, \text{lbs}$. To figure out the landing speed, we can look at this graph for full flaps from the performance manual:

We will use the touchdown speeds from the graph for the calculation, which are defined as

The touchdown speeds presented in figures A8-1 and A8-2 are the initial speeds
at the point of contact in landing. [...] The speeds represent 1.2 times power-off
stall speeds for each flap setting.

Hypothetically, one could touch down at the stall speed, but it's probably more realistic to use the factor of 1.2 times stall speed here.
However, these speeds are airspeeds. To calculate the ground speed, we need to subtract the headwind component. The video says it was up to 50 kt, but one cannot always get that. Some comments under the video note that a modern aircraft carrier can always achieve 32 kt, when travelling into the wind.
Let's plug in a few numbers to see how much kinetic energy we get:

Weight
Airspeed
Headwind
Energy

75 000 lbs
81 kt (Stall)
50 kt
4.3 MJ

75 000 lbs
97 kt
50 kt
9.9 MJ

75 000 lbs
97 kt
32 kt
19.0 MJ

85 000 lbs
97 kt
50 kt
11.3 MJ

121 000 lbs
104 kt
50 kt
21.2 MJ

130 000 lbs
89 kt (Stall)
50 kt
12.0 MJ

130 000 lbs
107 kt
50 kt
25.4 MJ

130 000 lbs
107 kt
32 kt
43.9 MJ

(Airspeed is touchdown speed read from the graph, or stall speed without the safety factor of 1.2)
While megajoules sounds like quite a lot, these numbers are surprisingly small, mostly due to the relatively slow ground speed after subtracting the headwind. For reference: an A380 RTO corresponds to about 1.7 GJ according to this answer.
Where did it all go?
Usually, most energy is dissipated by the wheel brakes, which should be more than capable of absorbing this energy (a single brake on the A380 can absorb 120 MJ). However, the C130 can also use reverse thrust during landing:

Reverse thrust is applied by moving the throttles from FLIGHT IDLE to GROUND IDLE, and then into REVERSE range in coordination with nosewheel steering.

(C130 Flight Manual - 2-55 - Normal Landing)
This could further reduce the energy going into brakes, but since the video shows the C-130 stopping very quickly, there was likely not much time spent in reverse.

If not too troublesome, you could edit the actual weights  used in the tests into this answer, as they are mentioned in the Dark Skies video linked in the question.

@Jpe61 Thanks, I hadn't seen those. I included them into the table.

No math in the "where did it go" section?) I added landing roll distances to the question.

@Pilothead - Even with the landing roll numbers I don’t think there’s enough info to determine the ratio of braking power and reverse thrust. What can be said is that the energy dissipated by the reverse thrust goes into accelerating air molecules while the energy dissipated by the brakes is transferred to the boat (except for the energy that is converted to heat in the brakes themselves and I suppose some to the abrasion of the tires)


text:
Do the engines still spool up when reverse thrust is selected, but the reversers are not deployed?
The recent question When a large commercial jet touches down, is it committed to stopping? referenced the crash of Red Wings Airlines Flight 9268, which overran the end of the runway at nearly full touchdown speed due to failure of the reversers to deploy. According to the Wiki article, due to a crosswind, the aircraft touched down on only one of its main gear. Even once the nose gear was on the ground, the right main gear strut had still not compressed. The pilots selected full reverse thrust when the nose gear touched down. Since the right gear strut had not been compressed, the reversers were not deployed, however, the engines still spooled up, resulting in forward thrust.
I understand the reasoning behind not deploying the reversers when one or both of the main gear struts isn't compressed. However, it seems to me that preventing deployment of the thrust reversers should also prevent spooling up the engines for exactly the reasons that the Red Wings flight crashed. Applying full forward thrust when the pilots have selected full reverse thrust seems like a very bad idea to me.
I'm wondering whether this behavior is typical of modern transport-category jets. That is, would, for example, a Boeing or Airbus aircraft prevent the reversers from deploying, but still spool up the engines (as the Tu-204 did in this accident) or would they also prevent engine spool up when reverse thrust is selected while thrust reverser deployment is locked out?
Am I missing a reason why still spooling up the engines would be desired when reverser deployment is locked out?

I don't know the systems on a Boeing or Airbus, but other aircraft do prevent it with mechanical interlocks on the thrust reverser lever until they are fully deployed.  However, they are often tied to specific switches on specific gear and it could unlock it if one (or more) gear are compressed even if they all aren't.  Even in systems designed this way, they are not foolproof: There have been instances where TR's were deployed, power was applied, and then they stowed due to another issue and left the engine producing full forward thrust.

Although not exactly the same thing you're asking you might be able to surmise an answer about Airbus from what happened in the crash of TAM flight 3054.
It was an A320 landing in Sao Paulo with a known inop T/R on the right engine. The pilots retarded the left thrust lever to idle to engage spoilers and then into reverse to engage left T/R but left the right lever in CLIMB. The autothrust turned off and due to the lever positions the left engine went into reverse thrust while the right engine went into full forward thrust causing them to veer off and overrun the runway and crash.
According to Airbus this had been a problem before so they changed the procedure for single T/R. It was previously to bring both to idle then only the working one to reverse. This was apparently confusing the pilots causing accidents so they changed it. The correct procedure is now to bring BOTH levers to idle then at touchdown, BOTH to reverse. Obviously if this is the procedure the A320 will not normally spool up the engine that did not have reverse thrust. The only reason it did was because the pilot left the right lever in the climb detent.
I'm not sure if this is true if the T/R's were PREVENTED from deploying due to the wheels not touching down but I would figure it would.
The way the A320 thrust levers work is that you release a catch then pull the levers aftward past the idle detent. (Not a pilot so correct me if I'm wrong here) Other a/c have a separate set of levers, for reverse thrust, sometimes hinged to the main thrust levers.
According to this illustration I found at this site the DC10 did have such a system to prevent the thrust to be increased until the reversers were deployed.
According to a BEA investigation report the B747 has a similar system:

The thrust reverser mechanism is commanded by one lever located on each thrust lever. In the ground position and when the thrust levers are at idle, it is possible to pull the thrust reverser levers back to the "interlock" position which corresponds to reverse. This unlocks the reversers and initiates their deployment. There is blocking in the interlock position when the reversers are in motion or incompletely extended. The use of full reverse thrust is possible when the "in transit" lights are off and the "unlock" lights are on (see appendix 2) to indicate that the reversers are deployed and locked. (BEA site)

For the regulatory aspects of this question.

§ 25.933 Reversing systems.
(a) For turbojet reversing systems—
(1) Each system intended for ground operation only must be designed so that during any reversal in flight the engine will produce no more than flight idle thrust. In addition, it must be shown by analysis or test, or both, that—
(i) Each operable reverser can be restored to the forward thrust position; and
(ii) The airplane is capable of continued safe flight and landing under any possible position of the thrust reverser.
(2) Each system intended for inflight use must be designed so that no unsafe condition will result during normal operation of the system, or from any failure (or reasonably likely combination of failures) of the reversing system, under any anticipated condition of operation of the airplane including ground operation. Failure of structural elements need not be considered if the probability of this kind of failure is extremely remote.
(3) Each system must have means to prevent the engine from producing more than idle thrust when the reversing system malfunctions, except that it may produce any greater forward thrust that is shown to allow directional control to be maintained, with aerodynamic means alone, under the most critical reversing condition expected in operation.
(b) For propeller reversing systems—
(1) Each system intended for ground operation only must be designed so that no single failure (or reasonably likely combination of failures) or malfunction of the system will result in unwanted reverse thrust under any expected operating condition. Failure of structural elements need not be considered if this kind of failure is extremely remote.
(2) Compliance with this section may be shown by failure analysis or testing, or both, for propeller systems that allow propeller blades to move from the flight low-pitch position to a position that is substantially less than that at the normal flight low-pitch position. The analysis may include or be supported by the analysis made to show compliance with the requirements of § 35.21 of this chapter for the propeller and associated installation components.

To activate reverse thrust you must move throttles back. This movement is physically prevented by the system that monitors if the airplane is in the air. But if something breaks then it's possible that the engine will provide direct thrust.

The behavior sounds exactly right, at least in theory. Once you touch down, there are two viable options: Either you perform a go-around (requires thrust) or you come to halt (which can be done with thrust reversers). Continuing at the same slow speed with the same limited thrust never is a good idea.

Normally, though, for a full stop, wouldn't you want the engines to remain idle until the reversers are deployed (using brakes/spoilers/drag to slow down until the reversers can be deployed?) You generally don't want the plane to be speeding back up while you're trying to get it settled onto the runway (as happened in the referenced crash.)

@reirab: Engine reaction isn't instant anyway.

I'm not sure if this answers the question..."would it spool up or not".

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Does an aircraft carrier have a VOR onboard?
I wouldn't think so due to the impracticality of VOR on a moving platform, but aircraft still need to navigate to the aircraft carrier. Would the radial shift around as the boat rocked around? The enemy could use the VOR to literally home in on the carrier as well.

"Apollo to Galactica.  Is your autoland offline?" The reimagined series (2004)

The USN is not the only navy with aircraft carriers.

Aircraft carriers use a TACAN, which stands for Tactical Aid to Navigation.  (The Navy is really good at making acronyms that don't follow normal conventions for abbreviation)
Functionally the TACAN is like a VOR with DME, providing bearing to and from the ship, as well as range.  The system is not susceptible to rocking motion, and least not perceptibly.  (and if you are already familiar with VORs, a VORTAC is a landbased co-located VOR and TACAN station.)
And true, an enemy could pick up the signal.  For this reason carrier airwings need to prove themselves proficient in EMCON (Emission Controlled) launch and recovery operations before being certified ready for deployment.  During an EMCON recovery crews are told where to expect the ship, what the expected recovery course will be, and  will be given a marshalling lat/long.
At the appointed time, and with no radio communication, formation flights will peel off the bottom of the holding stack, drop to break altitude,  head towards where the ship should be, and sequence themselves in with traffic in the pattern.  It is a pretty amazing thing when it works.

So an EMCON landing is like landing at an uncontrolled field except that there is zero radio communication. And the field is moving. And the weather may be horrible. And many if not most of the aircraft are low on fuel. And the pilots are all mentally exhausted. Good thing they're all well trained and know what to expect.

The Navy tends to avoid acronyms. They prefer abbreviations. You can find them in the DICNAVAB - Dictionary of Naval Abbreviations.

TACAN deals with the ship motion by having the antenna mounted in a gimbal so that it remains mostly level.

It is a pretty amazing thing when it works. - It probably is a pretty amazing thing when it doesn't work.

@Aleks G, generally no, when EMCON is broken is it usually just a needed radio call breaking the silence, and a collective feeling of disappointment that it wasn't as clean as it might have been.

@AleksG Imagine a formation peeling off from a holding stack, heading towards where the ship should be, and there's no ship there so they precisely glide into the water and disappear never to be seen again. At least, it would be funny if they were autonomous planes.

You'd expect they'd be using laser beam communications right now. They're pretty narrow, making it virtually impossible to listen in if you're in the " wrong" spot.

@user253751 You'd have at least some visual sense on the pilots' part.  If the ship isn't there, they will not just glide into the water, I'd hope.

@AleksG Another reason why it would be much funnier with autonomous aircraft. Perfectly stable, but it doesn't know the ship isn't there. I knew we shouldn't have gotten the intern to write the landing algorithm!

@MSalters, I can't see how lasers would be practical.  Flight cycles are typically 1.5 hours, with roughly 20 aircraft airborne, perhaps operating above a heavy layer of clouds, plus the ship will be within a few miles of where it plans and moving all the time.  How might the airplanes be tracked and use lasers to find the ship?

@MichaelHall I assumed that the meaning was laser-based communications with the ship on final. Though, at that point, I'm not sure why communication would be especially necessary.

@Gerry That thing is 330 pages?! It's amazing Navy personnel can tell the abbreviations apart...

OMG, I thought that was a joke!

@Gerry: How does that compensate for the antenna's lateral motion (tending to shift the detected azimuth clockwise and anticlockwise in an oscillating pattern) as the ship rocks from side to side?

@Sean To my knowledge it doesn't, but it's not really necessary. TACAN is for finding the ship, not a precision approach. As an estimate, if the antenna is 30 m above the roll center and the ship is rolling +/- 20 deg, the antenna will be translating about 30 m left to right. At a distance of 1 NM (fore or aft) that woud cause about 1 deg bearing change. At longer distances it would be hardly noticeable. Besides, I'm not sure it's all that relevant these days. Between GPS and secure datalink there are other way to find the ship than a UHF beacon.

There is visual communication during the last phase of landing between the aircraft and the ship. The ship is lit (somewhat). Other ships should be keeping the bad guys away from the carrier anyway.

@Gerry Many things on Navy ships are synchronized with the orientation of the ship. Synchros send yaw, pitch and roll information to things all over the ship, including radar antennas. I would expect TACAN to be similar.

@MSalters I suspect that lasers would not work very well in rain or fog, especially given that you don't want to dazzle the pilots (visible wavelengths) or damage their retinas (visible and IR wavelengths) when the beam direction is changed by water droplets. While there might be plenty of space on the ship for a receiver which can feed back to the airborne transmitter which way to aim, there would not be that space on an aircraft. And you really don't want a retroreflector on a combat aircraft.

Just wait until he finds out about Air-to-Air TACAN


text:
Why did the pilots in Top Gun: Maverick invert at the crest of the mountain before descending?
In Top Gun: Maverick, the pilots plan to perform a bombing run in the valley of a craterous mountain range in F/A-18s. At the crest of the first side of the crater, the pilots were instructed to roll to an inverted attitude, start their descent, and, then, roll back over. The in-movie explanation was something like "to allow you to maintain the lowest possible altitude", which was of concern to avoid SAM targeting.
The maneuver looked very graceful and cool, but is there any real-world merit to flying this way? Maybe you can descend more responsively this way?
I drew a little animation that may demonstrate better what I'm explaining. (F-18 credit)

Albeit the whole thing being very unrealistic, I do believe this would be the right thing to do as negative G's are much less tolerable to the pilots (and the plane) compared to positive G's. I'm gonna go out on a limb here and guess that the plane also has higher pitch ratio when pulling back (it can complete the maneuver faster), but I could be wrong.

It's probably because it looks cooler.

Depending on how close the target is to the mountain, I wonder if this maneuver gives the pilot a quicker visual of the target as they come over the mountain, since the target would first appear "up" relative to the aircraft. Wind screens on fighter jets typically have more visibility forward, up, and backwards rather than down, due to the nose occupying space in front of the pilot. Maybe being inverted allows you to look up in order to see the target sooner?

In real life, the mission briefing would not include instructions to roll at that moment to pull positive gs instead of pushing negative gs because any pilots qualified to fly the mission would already know that’s how to fly a modern fighter aircraft.

@ToddWilcox especially if there's a convenient film crew on the mountain top shooting a blockbuster movie :)

It’s to maintain a positive loading  throughout the maneuver.  The F/A-18E is load limited to +7.5G/-3.0G, depending on loadout and flight operations.  With a designed in factor of safety of 1.5, the airplane could potentially experience structural damage at negative load factors in excess of -4.5Gs.  That’s not going to be acceptable in order to hug the terrain over a mountain ridge after a 30 or 40° pitch up to hug the terrain, causing Mav’s F-18 to go sailing right over the target, still climbing, and straight into enemy air defenses.
And then there’s the pilot - the meat servo at the controls.  High negative G pushovers are quite painful.  Blood rushes into the brain causing redouts and potentially could damage the ultrafine capillaries of the cerebral vascular system.  Combat is definitely not a place where you want to have a intracranial aneurysm.  In addition, a hard negative G pushover followed by a hard positive G pull can result in a GLOC (G induced Loss of Consciousness) during the positive G pullup as your blood goes from being smashed up in your head to rushing into your abdomen and extremities.
And that’s probably about the only realistic thing I can think of in regards to that whole attack run.

But this would call for being inverted during some symmetric interval around the crest, when the acceleration is downward. OP said the pilots inverted at the crest. That is when the velocity changes from upward to downward, but the acceleration has already turned downward before that.

nanoman: You always need to add "this looks cool" factor to physics calculations of any movie scene - which probably affected the choice of this maneuver.

@nanoman: I think this is what is meant in the answer with "maintain a positive loading throughout the maneuver"

@nanoman That's something done with relative consistency throughout the film. You might note that they show the "highest Gs" during the climbout (which would have negligible load), not at the bottom where they would actually "pull up". It's also shown as if pilots would need to pull on the stick hard to keep climbing. Holywood at work.

@nanoman, with the responses that have been coming in, I've reconsidered where the roll occurred -- I'm not totally sure if it was at or before (to keep positive G loading) the crest. It was a minor detail in the movie.

@Mavrik, no, they didn't do it to look cool, that's how you cross a ridge if your goal is to hug the terrain while not busting a bunch of capillaries in your eyeballs.  But I get it, because Hollywood IS well known for embellishing things...

Which makes me realize the enemy could have nullified the whole attack by stringing cables along the entire perimeter of that tarn.  The resulting wirestrikes would have cut any airplane trying an attack like that to pieces.

@CarloFelicione If memory serves, that actually is a thing that has been done, no?  Although I seem to recall more in valleys between mountains where radar can't see.

Is (for example) -1g equivalently "bad" to +3g, or is it even worse? In other words, since humans are evolved for +1g, are negative g's being worse simply a matter of starting the "neutral" value at +1 rather than 0?

@eps a pilot is trained to physically combat positive Gs, through breathing and muscle clenching techniques and also through the wearing of a g-suit which compresses the lower body to force blood back toward the brain (where its been forced from).  Negative Gs see the blood rushing to the brain, and theres no tensioning, clenching or compression that can be done to stop it.  You “red out” due to too much blood in your brain - it freaking hurts too.

@eps similar to lifting weights, G tolerance is an acquired and developed skill.  From a pilots perspective, it’s just much more comfortable to pull positive Gs than negative G.  I have done -4 Gs in an MX3 aerobatic aircraft, which caused me a light redout (world went rose colored) but nothing else.  Professional aerobatics pilots will built up a tolerance as high at +10G/-7G in prep for airshows and aerobatics competitions.

Even when you’re not crossing a ridge, when you’re diving in an attack fighter jet such as an F/A-18 or F-16, you start the dive by inverting, pull to your dive angle, and then rolling back upright. Pretty much any time you change the direction of a combat jet fighter, you do it by rolling until your lift vector points in the direction you want the nose to go and then pull back on the stick. There’s a lot of nuance and some situations (like landing) are exceptions, but that’s the basic idea. Having the top of the jet facing where you’re turning to also means you can look where you’re going

@CarloFelicione "enemy could have nullified the whole attack by stringing cables" Yeah, and if the Empire could have just armored that ventilation shaft a little better...

@ToddWilcox spot on.. Either that, or wingover...

Think you mean ‘small thermal exhaust port’ there, Glen…

@MichaelHall I love your mention about capillaries.  I break eyeball capillaries around 4.5G so I've only broken them in positive Gs.  It reminded of them being called "bobber eyes" because it made one's eyeballs look like the classic style fishing bobber, white on top and red on the bottom.  I s'pose negative Gs would result in "Poké Ball" eyes.

@RomeoSierram a wingover isn't a very good ridge crossing technique if that is what you are suggesting...

Human negative G limits for pushing the nose down are about 3G, vs. maybe 9G positive G limit for pulling the nose up (or downward while inverted), very roughly, and as other answers explain, negative Gs right before positive Gs could be even worse.  Airframe design limits for the F/A-18 are about -3 .. +7.5 G, per @max's answer.
This is how real fighter pilots fly when following terrain, even when it's less dramatic than a mountain peak.  When cresting a smaller ridge, you might not roll all the way inverted, but rolling to 90 degrees as you approach the crest removes the vertical component of lift. And rolling past lets you pull some Gs to get the nose down while turning some.  (Or just go fully inverted if you want to keep your heading while dropping the nose by any significant angle.)
C.W. "Mover" Lemoine flew the F/A-18 for the US navy (and other jets for the USAF).  On his youtube channel, he posted cockpit video from his first low-level training flight (looking forward over his shoulder), adding voiceover to talk through what was going on and why.

[4:21] ... [my wingman was] very aggressive in doing the ridge-crossing and stuff which you just saw. Instead of bunting, instead of pushing negative G, you roll to get the positive G on the aircraft so you're not pushing over every time you cross a ridge.

On another ridge, at 6:13, he rolls all the way inverted to pull the nose down after crossing the ridge to see how much drop off there actually is.  That one didn't involve much of a change of heading, unlike some of the intentional turns at ridges in the training flight (which is another reason not to roll fully inverted).
This was his first low-level training flight doing stuff like this close to terrain.  He says at the start that he stayed a bit high while getting comfortable with the whole thing.  At 17:52, he rolls to about 120 or 135 degrees: getting to about 90 before reaching a ridge, continuing the roll a bit more after he can see past it.  And he's low enough to see details on trees before cresting the ridge.

If staying tight to the terrain over one specific big crest was critical to a real mission, you likely would roll inverted before the crest.  (Unless it was a really wide round curved hilltop, not a ridge at all.  If pushing the nose over to follow the curve would only leave the pilot feeling between 0 and 1 G, they might stay upright.)
I haven't seen Top Gun: Maverick yet, but if they only roll inverted at the crest, after pushing the nose down to horizontal, that's probably a movie mistake unless that part happened gradually.  Like a shallow curve and then a steep drop.
I'd guess that if you'd scoped out the terrain for a specific ridge and practiced in a simulator, you could be much more aggressive.  But if in real life you're just shooting footage for a movie, you're not going to risk your life more than usual.  So it makes sense that what you see in the movie is rolling and pulling down at the crest of the ridge, not much before.  Assuming that shot was 100% practical, and actually done close to terrain, not a separate shot of a jet composited onto a shot following terrain.

Re "but rolling well past 90 degrees as you approach the crest at least kills the lift from your wings" -- does the rolling maneuver itself actually "kill" the lift from the wings?  Or is it the fact that the pilot pushes the stick forward as he rolls, that kills the lift or makes the lift vector go negative?

(That's a real question, not a pure comment in disguise--) -- if a roll input causes the aircraft to roll about the aircraft's lateral axis rather than about the velocity vector, then the rolling motion itself will tend to cause any initial positive angle-of-attack to "flip" to the mirror-image negative a-o-a (roughly speaking), or vice versa, even if there is no change in fore-and-aft position of the stick. (Though the aircraft's built-in longitudinal stability will try to fight this effect to some degree, at least over the longer run.) But is that really how aircraft typically roll?

@quietflyer: Oh, fair point, what I meant wasn't what I wrote: I meant that any lift from the wings won't be in the vertical axis, when the roll angle is near 90.  (Because when I first wrote that, I had been thinking the roll I was talking about was only just past 90, but it's actually more like 120 or 135 degrees so there's a significant vertical contribution after all.  But I didn't edit that sentence.  Doing that now.)

Ok so basically you meant that the rolling motion changes the direction of the lift vector as seen by an observer on the ground, not by the pilot, I take it.  That's the way it intuitively seems to me like things "ought" to work (but see previous comments for some possible complications).   I don't (and never have or will) fly high-performance jet-fighters and the subject of these roll dynamics is fascinating to me-- I think there are some possible nuances that may often get overlooked--

@quietflyer: I found a way to rephrase which hopefully avoids implying anything that might be wrong.  I didn't really like the way I original wrote that paragraph.  The point you raise is interesting, but not one I want to get into in my answer, so I'd rather just sidestep it.  As for vertical lift created by the airframe slicing sideways through the air, yeah maybe still some.  As for negative angle of attack after rolling still creating positive lift while inverted, yeah I expect holding the stick back at all would maintain positive AoA, and fighters have huge pitch authority esp. at low alt

Positive G vs Negative G.
The pilot and aircraft can sustain greater pos G than neg. This would allow them follow a sharper arc going over the top, and stay closer to the landscape.
Of course, an actual fighter pilot will have more insight into this.

Virtually every aircraft is designed with a positive-G limit that is between 2 and 3 times the negative G-limit.  Most FA-18 models have a positive G limit of 7.5G and a negative G limit of -3.0G.
For comparison, a typical light general aviation aircraft like a Cessna 182 will have an envelope of +3.8G to -1.52G for what are called "normal category" maneuvers.  Some, but certainly not all civilian aircraft have an additional approval for specific maneuvers between +4.4G and -1.76G, but only under a more limited set of weights.  A 1,500 pound plane puts the same stress on the wing and tail structures at 4G as a 2,000 pound airplane places on them at 3G.  (6,000 pounds of force in both cases.)
These limits are used as part of the design process, so the aircraft's elevator is designed with more nose-up authority and less nose-down.  And further, when the pilot is trying to get into a high-G maneuver quickly, he/she wants the lift vector of the wings to be adding to, not fighting against, the entry into the maneuver.  So in maximum performance maneuver, a fighter pilot rolls the aircraft into whatever orientation aligns the wing lift vector with the desired performance.
So in the case of a high-G maneuver, it's not about pilot preference or what's "easiest" on the airframe, it's that the aircraft's design may not make a -7G maneuver possible.
( P.S. I don't mean to sound critical, but please keep in mind that on SE there is a difference between an answer that directly answers the question and a comment that provides commentary within the context of the question.  I suggest that the other answers provided would have been more suitable for comments.)

It's about pilot preference to the the extent that the body also has a far greater tolerance for positive Gs than for negative.  (as Carlo pointed out in his best answer...)

Also worthy of mention is that the pilot may have a significantly better view of the terrain while inverted.

Yup, the movie doesn't mention it but once they go over the ridge they have to not only acquire the target (which I believe is entirely by camera, orientation doesn't matter) but given the low altitude of the drop the bomb has very little cross-range capability, the plane can decide when to drop but unless the pilot basically overflies the target the bomb won't be able to adjust.  (Not that it would ever happen like that--the first bomb would have killed the plane that dropped it.  The second explodes underground so it might be survivable.)

+1  I remember reading an interview with an A-10 pilot, many years ago, who said they did this to keep low and spot targets.

Inverting gives you two free negative G's
Take a Southwest 737 in cruise.   It has 1 gee operating on it from regular old gravity.  Then it has another gee from the normal trim of its wings.
These cancel out and it holds altitude.
Now if the aircraft inverted while doing their best to sustain "perceived" 1.0 gee in the cabin, those two gee-forces would stack instead of cancel out.  Thus giving a -2 gee downward force, whilst in the cabin, the passengers would feel the entirely normal one gee.
A skilled pilot might not even spill the passengers' drinks.
Cresting the peak of a hill, you need negative gee's to keep hugging the terrain.  Imagine you have been climbing the hill at 3 gees and need -5 gees to crest the hill, then back to plus 3 gees to hug terrain.  If you make that extreme transition from +3 to -5 in level flight, you'll have extreme change of acceleration, or colloqually "jerk", which will break the airplane and the pilot.
But if you simply invert, you get a free -2 gee and you simply continue pulling 3 gees, netting out to -5 gees.   It's a smooth transition with no jerk and no disorientation.
Probably not a perfect "aileron roll at 3 gees" because of terrain... but you are adjusting in a limited range of say 1.5 to 3.5 gees, which is manageable and you do it all the time as a fighter pilot.
A Youtube channel called Grim Reapers regularly simulates aerial feats, real or hypothetical, relating to recent wars, and often penetrating first-rate air defense networks such as Aegis, S-400 or Pantsir.   They regularly make use of this technique.  On one fictional helicopter raid, all were picked off by the S-400 system except one pilot who made great use of this technique. With a helicopter.

There might be something wrong with the gee math in my example, but you know the concept is correct. Feel free to correct rather than downvote.

Negative Gz's are dangerous and Navy pilots are trained to reduce or eliminate negative Gz's.  Look up the push-pull effect and you will understand why they went inverted over the ridge-line in order to accomplish the mission set.

Not sure that this adds anything to the existing answers, and saying to "look up xxx" isn't really providing information. Answers here are expected to answer the question, rather than suggesting what to look up in order to find the answer.


text:
Does the F-22 (or any other stealth aircraft) carry chaff?
Considering it is nearly undetectable on radar, it seems like it wouldn't have to worry about radar guided missiles; in addition to this it is sure to have sophisticated jamming capabilities. So why carry chaff? Just weight.

wikipedia says it carries flares for IR guided missiles

@ratchetfreak Flares =/= chaff

my point exactly.

Combat (particularly fighter) aircraft typically carry both chaff & flares. They aren't mutually exclusive. Wikipedia isn't the most credible source for the newest 5th generation stealth fighter either haha.

Chaff doesn't weighs much; the lighter it is, the longer it stays in the air. The F-22's payload is several tons.

Wouldn't the chaff/flare dispensers present a radio frequency signature management problem, resulting in the deletion of them from the design?

@brinnb I imagine an alternate delivery system could be devised with a minimal radar cross section - all that's really needed is a hole through which the flare (or chaff) could be expelled, perhaps by compressed air...

You raised a really good point putting "nearly" in your question. The techniques evolves, it is reasonnable to assume that stealth today is not the same as stealth in few decades. Moreover, detection capabilities differ depending on the ennemy. Thus wise to carry state-of-the-art jamming device AND chaff. As pointed out by @ChrisW carrying chaff is not such a constraint.

@ManuH the chaff itself isn't that heavy, but combine it with the equipment needed to launch it, the controls in the cockpit, the wiring, electrical circuits, probably automated systems as well integrated with the threat detection devices, it all adds up both in bulk and weight.

If nation A has top-secret stealth aircraft then surely it should assume the worst-case scenario, which is that nation B has a top-secret way of defeating their stealthiness. Because if nation B did have that then it would make every effort to keep it secret.

It might be worth mentioning that the capability to carry and the decision, or need, to carry are different things.  I can't answer for the new generation, but older models the dispensing system was capable of handling cartridges with either chaff or flares. (cartridges were interchangeable) The actual load carried, (or mix of loads) was decided based on the threat.

It certainly seems so.
Stealth aircraft are not invisible to radar, only less visible.  Under certain conditions the adversary will be able to get a radar lock:

At close distance
With very high (focused) radar energy
At certain angles of incidence

Given these risks and the low weight of chaff, it wouldn't be be wise to not carry it.
One source that claims the F-22 carries chaff: http://science.howstuffworks.com/f-22-raptor8.htm

It releases flares to confuse heat seekers and sends out chaff, small pieces of reflective material, to disperse radar waves and confuse a radar-guided missile's tracking system.

It's even been used in the field, according to The Washington Post:

the [F-22s] shot flares and even chaff — clouds of metal meant to confuse radar systems but possibly used in this case as an additional visual warning.

That said I did read or here somewhere (not sure where, but it was reliable-sounding source) that B-2 does not have chaff.

Good point. Keep in mind that chaff and jamming work together. Radar is beaten when the noise exceed the real signal. If you get a unjammed bounce off the chaff and a jammed bounce off a stealthy F22, the radar is quite likely going to give up.

@JanHudec This B-2 documentary mentions that the B-2 does not have chaff nor flares.

@florisla - There are strategic, technological and tactical reasons for this. The biggest reason is that the B-2 doesn't maneuver very well, and chaff/flares really only work in combination with high-G maneuvering to cause the missile to fall behind the aircraft's flight path. The B-2 was also designed to penetrate USSR territory unseen and launch nuclear cruise missiles at strategic targets. In that doomsday scenario, surviving to launch is the only goal; once the bomber's empty it might as well kamikaze, as its home base and the entire chain of command will all be gone before it returns.

The F-22, while stealthy, is also designed to maneuver and dogfight, and so the added weight and bulk of the chaff/flare dispenser is considered necessary to help keep a $185 million air superiority fighter in the air fighting for superiority, instead of falling to the ground in pieces.

I can't validate beyond a personal anecdote, but the F-22 carries a chaff/flare system - I know this, because I've touched a prototype launcher. Back in high school (mid-90s), my school worked with Lockheed Martin via US First. One of the engineers we were working with had it on his desk. It was a surprisingly small device.

Also, jet fighters are designed to be in service for decades. One could assume detection technology might improve during its lifespan.

@keithS I cannot speak to todays war plans, but back in the 1980's SAC had two scenarios, a surprise attack and couple of days build up to open war. For the surprise attack the aircraft we kept on alert could be off the gourd in a bout 5 minutes.. This was often practiced. For the couple of days build up, we would arm every plane then send out teams to set up alternate bases at civilian airport, old military bases and even log stretches' of the interstate. This so we could recover and rearm returning planes for a follow on strike. This usually only minimally practiced.

Yes. The F-22 carries a pair of AN/ALE-52 countermeasures dispensers capable of carrying combinations of chaff and flare cartridges.  The dispensers are located on the underside of the fuselage aft of the main weapons bay.  Similar to the gun muzzle, the dispensers are covered by a pair of fast acting fairing doors, which snap shut quickly after use to minimize disruptions in the radar cross section of the aircraft during use.


text:
What frame of reference should I use for my Simulink simulation of the F/A-18 Hornet?
I want to simulate the Equations of Motions for F/A-18 Hornet (12 states & 6 DOF) on MATLAB using Simulink.  I have the Aerodynamic model, from which I will get CL, CD, CY, Cl, Cn, Cm that are flight conditions dependent.
Now, the forces and moments calculated from these coefficients will be in Wind Axes because these coefficients are defined in that coordinate system only (correct me if I am wrong). For using these Aerodynamic Forces and Moments in Body Axes, do I need to convert them into body axes by multiplying a Direction Cosine Matrix, that converts vectors from Wind Axes to Body Axes? Or should I use the Wind Axes computation?
Assume, I took the first option and converted the forces and moments into Body Frame. But, there are terms like $p$, $q$, and $r$ in the computation of the coefficients mentioned above. These terms are defined in the body frame. So, should I use these values of $p$, $q$, $r$ that are in body axes and compute coefficients in wind axes? Or should I convert them also into wind axes before using them in the computation?

PS: Reference for the Aerodynamic Model of F/A-18 Hornet:

A. Chakraborty, P. Seiler, and G. J. Balas, Susceptibility of F/A-18 flight controllers to the falling-leaf mode: linear analysis, Journal of Guidance, Control, and Dynamics, Vol. 34, No. 1, pp. 57–72, 2011.

It's very prudent of you to be concerned about the axis frames in which different forces/motions are defined. It's a common mistake to ignore the fact that aerodynamic coefficients are normally defined in wind axes.
The standard approach is to calculate Equations of Motions in the body frame; this is how they are usually defined. For this, you'll indeed need a wind-to-body matrix (DCM). This is a simple matrix because it involves only two angles (beta and alpha), and these angles are normally small to be concerned about gimbal lock (the model is probably not defined where they are not small). Then you

Calculate the forces/moments that are defined in wind axes and rotate them to body.
Likewise, calculate forces and other things that are defined in the ground frame, such as gravity and wind per se, and rotate them to body. (It makes sense to use quaternions for that rather than DCM, but this is another matter).
Finally, you'll have forces that are defined directly in the body frame, such as thrust.

Now you can integrate EoM with all the data available in the body frame.
Regarding p, q, r (and such) used in calculation of the aerodynamic forces, these are usually body angular velocities that you can take directly from your EoM. However, you should double check if the aerodynamic model requires something else; this should be explicitly defined. Note that if you do need to rotate angular velocities, you can't use the same DCM but need to use the matrix that preserves the total angular momentum.

Thanks for clearing my doubts!

do you know how to model the Propulsion system of the FA-18 Hornet for the above simulation? Do you just assume that the thurst line passes through CG (no moment) and thrust is only in X direction? Please attach some references, if you have any for thrust modeling.

Well, that's an entirely different level. Now you are talking about the physical model itself. It depends on how much accuracy you need and how much data you have. The direction and origin of the thrust vector is part of the aircraft/model data. I don't have it, but I'd guess that for F/A-18 it's pretty close to neutral (along X through CG), as long as you don't simulate single engine operation. For the first approximation it might do, but if you simulate variable mass (ordinance/fuel), the CG may shift laterally, while thrust shouldn't.

Thanks for the info!

I'd beg to differ and in his particular F18 model, CL CD CY are defined in the stability axes, and Cm Cl Cn are defined in the body axes. The only publicly available source for aerodynamic coefficients is the NASA F18 HARV.

It's not true to say that those aerodynamic coefficients are defined in wind axes. To be specific to your F/A-18 model, which usually originates from the NASA F18 HARV model, CL CD CY are defined in the stability axes, and Cm Cl Cn are defined in the body axes.

Please refer to the following NASA document for a better description of the Equations of Motion and control schemes used in their simulation: https://ntrs.nasa.gov/api/citations/19920024293/downloads/19920024293.pdf
Here's the NASA F18 HARV wind tunnel data where you can get all the coefficients of force and moment: https://ntrs.nasa.gov/api/citations/19900019262/downloads/19900019262.pdf


text:
What fighters have both 1 & 2 engine versions?
Northrop originally built the F-5 "Freedom Fighter" (and T-38 "Talon") with two engines. Later it was re-fitted with a single F404 engine from the F/A-18, re-designating it the F-20 "Tigershark" to avoid confusion. The first F-5A and T-38 flights took place in 1959.
[Addendum: There is no real contradiction, the T-38 "Talon" was a two place trainer version of the F-5 "Freedom Fighter". Most aircraft don't re-designate between one and two seat versions, but Northrop or the military did in this case.] See the history of the F-5 YouTube Video
"Though the USAF had no acknowledged need for a light fighter, it did procure roughly 1,200 Northrop T-38 Talon trainer aircraft, which were directly based on the F-5A." https://en.wikipedia.org/wiki/Northrop_F-5
"The F-5G was an advanced single-engined variant later renamed the F-20 Tigershark." https://en.wikipedia.org/wiki/Northrop_T-38_Talon
Have any other fighters (world wide) offered a fighter with one and two engine models?

Twin engine F-5 "Freedom Fighter".

Single engine F-20 (F-5G) "Tigershark". Wikipedia

There were only 3 F-20's ever made, two of which crashed. The DoD decided to go with the F-16 (aka Lawn Dart) after that.

Basically for the same reasons any aircraft would have multiple engines. There may also be a difference in maneuverability but I'm not necessarily qualified to comment on that.

The T-38 was the Talon, the F-5 never had that name.

The F-5A/B are the "Freedom Fighter", the F-5E/F are called "Tiger II".

While they look very similar, the T-38 is not the trainer version of the F-5. The wings are different and the fuselage is more curved on the T-38.

According to the cited YouTube video The T38 is a variant of the F5. The video goes into some interesting major quick bolt add-on such as a reconnaissance and gunship version of the front half of the aircraft. This airplane was truly a jigsaw approach to a light weight fighter.

@ qqjkztd - +1 great answer, I was thinking jets but yes that certainly counts!

Not a fighter, but didn't Avro develop a 4-engine version of the 2-engine Manchester bomber, during WW2?

Actually, the T-38 came first, follows by a export fighter derivative called the F-5.  Both are based upon Northrop’s N-156 design study,

Not directly related but A5 has a 3 engine version proposed.

@jwzumwalt the F-5 was created out of the T-38 when the US government identified the need for a cheap low tech combat aircraft that could be supplied to client nations either too poor or not trusted enough to be supplied with the then current line of front end US combat aircraft.

The F-20 wasn't a single engined F-5, it was a completely new aircraft designed out of the F-5. It's similarly related to the F-5 as the F-100 is to the F-86.

"Though the USAF had no acknowledged need for a light fighter, it did procure roughly 1,200 Northrop T-38 Talon trainer aircraft, which were directly based on the F-5A." https://en.wikipedia.org/wiki/Northrop_F-5

That would be impossible since the T-38 began its life in 1959 as an N-156T derivative and the F-5 in 1962 as an N-156F derivative.  Wikipedia screwed up there.

The Dassault Balzac V was a VTOL testbed with nine engines. It was a modified Mirage III which had one engine.

The Republic F-84F Thunderstreak was a fairly normal early jet fighter, powered by a single Wright J65 turbojet engine.
Its mutant offspring, the Republic XF-84H "Thunderscreech" was a experimental turboprop fighter, powered by an experimental Allison T40 engine, which consisted of two Allison T38 engines, joined through a common gearbox.
This was a twin-engine aircraft from the maintenance point of view, since there were two sets of turbo-machinery to go wrong. It looked like a single-engine aircraft, and flew like nothing on earth, owing to the strange effects of two sets of supersonic shockwaves from the contra-rotating propellers hitting the wings several times per second. Its first test pilot refused to have anything more to do with it after the first flight; the second test pilot flew it eleven times, but Republic did not submit the aircraft for USAF trials.
Edit: The reason for doing this was that early jet engines were very fuel-thirsty, while turboprops got much better mileage. The propeller was supersonic because that was necessary to absorb the power in a small enough propeller to fit within the aircraft's ground clearance. It wasn't remotely a good idea, but it may have shown the USAF that jet-age aircraft development was harder than it had been in the piston-engine era.

Very interesting. Yes I would say that counts too!

I don’t know if you would call the XF-84H an option for a twin engined version of the F-84 airplane.  It was an experimental prototype used to study the possibility of a supersonic propeller driven fighter which never went into production.

Who is the heck would even dream of such an abomination?

@RonJohn: Explained some of the reasons.

@CarloFelicione it's just as much a twin engined F-84 as the F-20 was a single engine F-5 :)

Kind of but really not.  The XF-84H was a research aircraft and never really intended for production whereas the F-20 was in a production ready state and was being marketed by Northrop for sale to allied nations.

@CarloFelicione they fully intended to build the F-84H or a close derivative, until they actually tried it. I think there even was an MOU for a production batch signed before the XF even was built. That was the norm at the time. X meant prototype as much as research.

Well, the F-20 is a different airplane from the F-5, albeit an evolution of the original Northrop N-156 design which developed into both the F-5 and F-18 families of airplanes, so no aircraft model was ever offered with the option of having one or two engines.
Ideally fighters are more suited toward the single engine configuration as powering one with multiple engines invariably adds more complexity, systems to manage and weight.  The only justification for which is an increased thrust to weight ratio on a larger, heavier airframe.  I’d guess Northrop would have dearly loved to have the F-5 powered by a single GE J-79 turbojet given its for the time supreme thrust to weight ratio, but I don’t know if the engine was available for export back at that time.  The F-5 was also an export fighter derivative of the T-38 airplane which was currently powered by a pair of GE J-85 turbojets, originally designed for expendable use powering cruise missiles, which offered the Talon good performance and were available to sell to other nations.
Twenty years of both fighter aircraft and engine development provided the F-20 design team with the new GE F-404 engine, comparable in performance with the J-79 but with 6000 fewer parts and greater thrust to weight ratio, highly resistant to compressor stalls, and greater reliability than the older J-79.  This along with the availability for export made the F404 the preferred choice on the F-20; there was never any attempt to make a multi engine variant of the F-20.

Do you have a citation to back up this claim? This YouTube documentary says they are the same plane... https://www.youtube.com/watch?v=AvDfs6s4tbA&t=10s

The doc describes the F-20 as “the final development of the F-5 family” ie a development which became a new aircraft.  The YF-17/F-18 Family is also a development of the F-5 family of fighters.  The F-20 did begin its design life as the F-5G but was given a new designation of F-20.  While it does have a similar configuration to the original F-5, it’s a different aircraft underneath.

@jwzumwalt just another case of youtube being wrong I'm afraid.

AFAIK the engines were chosen for the F-5A based on cost and performance requirements. The requirement was for a subsonic or transonic combat aircraft with low cost and maintenance requirements, for distribution to nations with no qualified personel to maintain front line US combat aircraft and/or for which export of such was not allowed. That's why the radar installation and provisions for guided weapons were also stripped out (though some F-5A variants like the Canadian and Dutch ones could carry some guided weapons).

Lacks supporting citation - just an opinion.

"Though the USAF had no acknowledged need for a light fighter, it did procure roughly 1,200 Northrop T-38 Talon trainer aircraft, which were directly based on the F-5A." https://en.wikipedia.org/wiki/Northrop_F-5

Sorry, But Wikipedia doesn’t know what it’s talking about in that case.  The Northrop N-156 design evolved into the T-38 first, then an N-156 derivative became the F-5 family.  Wikipedia while pretty good is not always accurate.

Technically, several single-engine aircraft has been either designed or modified to utilize a second rocket engine at some point during their lives.
For one example, see the Lockheed NF-104A
The Soviet WWII era Su-6 and Su-7 would examples of a piston engine single (the Su-6) being re-equipped with a hybrid piston and rocket propulsion system.
It could also be argued that the Mirage IV fits this criteria, as its design was derived from the existing Mirage IIIA.

That's true, I hadn't thought of that. I guess any single engine aircraft with jato would also be in a gray area.

and then there's the ill fated Balzac, another multi engine Mirage variant. Twin Mustang fits the idea, and the various German attempts to mate 2 bombers at the wing to create a larger bomber.

The UH-1 Huey and AH-1 Cobra/SuperCobra could also be considered examples. Initial versions used a single engine, while later models use twin engines to drive the main rotor.


text:
Why must some aircraft get permission to start their engines?
Some larger airliners must get ground control permission before starting their engines.

Is this based on size? Does a Learjet need permission?
Is this just turbines? For example does a DC3 or DC6 need permission?
How would a pilot find out? Does the airport supplement or some other document show when it is required?
Are there exceptions such as jets parked on remote ramps, not at gates, non-commercial?

I know from flying small recips that I never needed permission anywhere in the US or Canada - but certain airliners did.
This airport in Australia says they enforces their right for the engine start. Part of the authorization comes from environmental noise control.  townsvilleairport (Australia).
Van Nuy - US, "All  aircraft  with  engines  capable  of  producing
more  than 14,750  lbs.  of  thrust must  be  pushed  back  or  towed  onto  the  taxiway  prior  to  engine  start." (Airport ops) (Of course you need ground approval prior to entering a taxi way...)
When working at Anchorage International, Alaska with Continental 737-757, we had to contact ground control. We could start the engines at the gate while doing maintenance; there were no ground crew around. We started the onboard APU, turned on the red strobes, got permission from ground, then started the engines. I never read a document, this info was relayed to me by my supervisors.

I wonder if you are confusing communication with ground control and ground crew. Ground control are the nice folk in the control tower who authorise movement on taxiways, and don't care when the engines are started. Ground crew however are airline personnel, and do care when the engines are started because they might get sucked in to one! One of them is usually communicating to the pilots during pushback via a headset connector, and sometimes in videos you might hear the pilots asking them if everyone is clear before starting the engines.

Airliners usually push back from the gate and then start their engines when well clear.
If the aircraft has an unserviceable APU, a ground based power supply must be used at the gate for the start. Permission(coordination) is required so that their are no hazards for ground crew and vehicles in the gate area.
After one engine is started at the gate, the aircraft is pushed back. The next start, still needs permission(coordination) because the operating engine usually has to increase it's thrust above idle in order to have enough bleed air pressure for the "Cross Bleed Start".
EDIT:  "Permission" may be not be mandatory at many airports, so "Coordination" with Apron Control, Ramp Control, or Ground Control may be a better description.

Is the tower responsible for making sure personnel are clear? That seems like something you would coordinate with the crew chief

The ramp crew doesn’t know which aircraft are going to be taxing behind the aircraft

Is engine start generally assumed to be part of pushback clearance?

I'm not sure that "permission" is the correct term. I think its a function of most airliner crews can't see behind them so someone with a good vantage point can assist in making sure they are clear, but I'm not familiar with any regulation that requires "permission" from ATC to start engine on ramp. If there is such a regulation it would be very enlightening to see it. I used to work on ground crew for 747, 767, 757, and another others at UPS and our ground crews handled clearing pilots to start the engines (i.e. making sure all were clear). Of course we did have our own private ramp area.

Two separate things:

you get a “clear” for engine start from the ground crews (either handsignals or via heatset/intercom) to ensure the area ahead/behind the engines is clear. Mind you, most cockpits you cannot see the engines, let alone the area behind them.
you get a “clear” for engine start from ATC via radio, though (much like the taxi instructions) this is not a clearance per se. It is more an ‘agreement/acknowledgement’ with ATC that you will be moving soon enough (they know it slightly better when you will be taxing) so you might go ahead and start the engines. They might need to keep you from moving another 10-15 minutes (departure slot, other trafic taxiing by,etc) at which point I’d preffer to wait with engines stopped.

It gets a little bit more complex if there is a pushback involved, especially at airports where ‘start’ is cleared on DLV frq and ‘pushback’ is cleared on GND frq, but most of the times the two clearances mean the same thing: the airport is ready to handle your traffic.